The present invention relates to dental implants which are adapted to be anchored in bone tissue to secure dental prostheses within the mouth. The invention also relates to methods for installing such inserts into bone tissue.
It has been proposed to secure dental prostheses within a patient's mouth by affixing the prostheses to dental implants which are embedded directly within the jaw bone. The implants and embedding techniques heretofore proposed and utilized in practice have not met with universal acceptance among dental practitioners due to considerable limitations regarding the placement of the implants within the mouth and/or uncertainties as to the useful lifespan of the implants.
In that regard, one currently employed technique involves the use of an implant 1 (depicted in FIGS. 12-14 of the accompanying drawings) having external threads 2 by means of which the implant is to be screwed into the bone tissue 3. A bore is first drilled into the bone tissue, the bore having a diameter less than the maximum diameter of the threads of the implant. By screwing the insert 1 into the bore, the threads 2 cut into the bone tissue 3. Variations of such a technique are disclosed in U.S. Pat. Nos. 3,499,222 and 4,431,416 and British Pat. Specification GB No. 1,291,470 published Oct. 4, 1972.
However, such a technique can result in so-called "saucerization" wherein some of the bone tissue does not grow back against the implant, i.e., areas between the bone and implant are occupied by soft tissue 4 which does not contribute appreciably to the anchoring of the implant. In cases where implants have been installed in the lower jaw bone, saucerization has been observed to occur at the upper end of the bore. Such saucerized areas can become infected, leading to a spreading of the saucerization along the implant whereby the securement of the implant is significantly undermined.
Although the exact causes of saucerization are not yet known for certain, it is the belief of the present inventor that because of the harsh treatment to which the bone is subjected during implant installation, the rate at which the bone is able to regenerate and grow against the implant is deterred to such an extent that soft gum tissue 4 is able to grow into and occupy the space between the implant 1 and the bone 3 at the open end of the bore. That deterred growth rate may result from a number of factors, including the severe traumatizing of the bone as it is cut and fractured along the region 3A by the self-tapping threads of the implant, and/or the likelihood that the bone is deprived of blood at least at that portion of the bone located at the top of a bore in the lower jaw since blood tends to gravitate away from that bone portion. In that regard, it is well known that blood, once it clots, will develop fibroblasts and osteoblasts which promote the development of bone tissue. By depriving the bone tissue of an ample blood supply, the regeneration and regrowth of bone tissue will be retarded, thereby permitting soft gum tissue to enter the area between the implant and bone. If such saucerization occurs and spreads along the length of the implant, the lifespan of the implant can be significantly shortened.
Some dental practitioners install more than the required number of implants into the bone, whereby the extra implants are kept available as "spares" in the event that the active implant(s) become dislodged from the bone. Besides being more expensive and inconvenient to the patient, that practice is only feasible in cases involving the installation of a relatively long prosthesis wherein the space between the active implant(s) is large enough to accommodate the "spare" implants.
In order to augment the anchoring of the implant, the implant can be provided with a hollow front end which receives a core 5 of the bone (FIG. 13). Such an expedient, disclosed for example in U.S. Pat. Nos. 3,499,222; 4,180,910; 4,359,318; and 4,431,416, involves the cutting of an annular kerf in the bone tissue to leave a cylindrical core of bone surrounded by the kerf. The open front end of the implant receives the core as the implant is inserted into the kerf. The wall of the implant surrounding the core is provided with holes 6 to enable bone tissue to grow through the holes. In the case of implants which include self-tapping screw threads 2, however, the screwing-in of the implant may impose sufficient lateral stresses on the core to cause the core to be broken-off at its base. Even if the core is eventually able to regenerate and regrow, the overall anchoring process could be delayed.
Furthermore, the act of screwing-in a self-tapping implant requires substantial torque imposed by means of a ratchet wrench which mates with a wrench-receiving socket 7 of the implant (FIG. 14). However, such wrenches are relatively large and cumbersome and require a relatively large amount of space to accommodate tool manipulation. Consequently, such implants are not suitable for use in the back portions of a patient's mouth where there exists little room to turn the handle of the wrench, or in narrow areas between existing teeth which are not large enough to accommodate the head of the wrench.
In addition, the fracturing of bone tissue caused by the screw threads renders the implants unsuitable for use in areas of exceptionally thin bone. Therefore, in cases where a patient's bone has been diminished in thickness due to surgical operation, injury, etc., the use of a screw-in type implant may not be feasible.
Therefore, it will be appreciated that for at least the above-discussed reasons, the overall utility and versatility of screw-in type implants has been significantly limited.
It has also been proposed to pre-form female screw threads in the bone by means of a thread-forming drill, rather than by the use of self-tapping threads on the implant. Such a technique is disclosed in a article entitled "A 15-Year Study of Osseointegrated Implants in the Treatment of the Edentulous Jaw" by Adell, Lekholm, Rockler, and Branemark, published in the International Journal of Oral Surgery, Munksgaard, Copenhagen, 1981, Vol. 10, pp. 387-416. In that technique, the thread-forming drill does not leave a core in the bore to augment the anchoring action. Also, the drill is designed to form a widening at the mouth of the bore to accommodate the insertion of an implant having an enlarged collar at is outer end. In practice, saucerization results from the formation of such an widening, and the saucerization gradually spreads, e g., by about 0.10 mm per year along the length of the implant according to the above-referenced article. Once a sufficient length of saucerization has occurred, the implant will be inadequately anchored. It should further be noted that even though the female threads are predrilled in the bore, the threads on the implant are intended to be partly self-tapping. Thus, a wrench is needed, whereby the areas of the mouth in which that implant can be employed are limited, and a certain amount of bone fracture and bone trauma will likely occur during installation of the implant.
It is, therefore, an object of the present invention to minimize or obviate problems of the types discussed above.
A further object is to provide dental implants and methods for installation thereof which minimize bone fracturing and traumatization and resist the occurrence of saucerization.
Another object is to provide such methods and apparatus which promote the regeneration and regrowth of surrounding bone tissue.
A further object is to provide such methods and apparatus which enable the implant to be installed in confined and narrow areas of the mouth and in thin bone tissue.
Yet another object is to enable an implant to be installed without the need for cement or wrenches.
An additional object is to promote the retention of blood and blood clots adjacent the cut bone tissue to promote the regeneration and growth of that bone tissue.
Still another object is to provide a hollow implant which fits over a bone core and which can be installed without an appreciable risk of breaking the core.